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Marty
Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 53166
Experience:  Bosch Certified Master Tech. Over 35 years experience.
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Marty,while checking fuel pressure on a 1986 560SL, because

Customer Question

Hi Marty,while checking fuel pressure on a 1986 560SL, because of a no start condition,the pressure gauge maxes out at 145lbs. According to my info the pressure should be 90psi at cranking. Would this indicate a failed pressure regulator or am I overlooking something?Thanks,Nick
Submitted: 1 year ago.
Category: Mercedes
Expert:  Marty replied 1 year ago.

Hi Nick, I need to know how you are testing the pressure. Can you send me a picture of your pressure gauge set up?

Customer: replied 1 year ago.
Here are some pics.
Expert:  Marty replied 1 year ago.

Hi Nick, I need to see how the gauge is connected to the car.

Customer: replied 1 year ago.
Marty,the gauge is connected right off the fuel pumps at the rear of car.
Expert:  Marty replied 1 year ago.

Hi Nick, if you are just checking the pump output pressure at the rear you will have 145 psi. To check the system pressure you will need to perform the test at the fuel distributor.

Expert:  Marty replied 1 year ago.

What is the history of the problem? Will the car run on ether?

Customer: replied 1 year ago.
Hi Marty,about 3-4 months ago my customer complained about long cranking cold starts.He lives in another city so the car went to a shop in his town and they replaced the fuel filter. That accomplished a new filter and nothing else. The owner then brought the car to us,a***** after a long crank start. He said it ran fine after a few minutes to warm up. I do not believe this has anything to do with the cranking issue but the security system on this car has been partially disconnected due to it being so sensitive to engage. The security horn (remove wires from horn) and flashing lights (removed 2 fuses from little junction block) have been disconnected but there is still a dull clicking noise coming from behind the glove box every time we attempt to start it. We suspect a relay. If so,can we just remove the relay? He wants it disconnected. I have ordered a service manual. The fuel pressure was dead heading at the gauge so we have to check it in line at some point. You mentioned the fuel distributor. I will check there for a port? Yes,we can start the car on a few shots of ether and it will run fine after a few minutes of erratic idling. Just a guess but could it be a cold start valve. I'm not sure how to check the valve without a manual. I just started the 560SL, with ether ,and let it warm up. It will restart without ether as long as it is warm but not when cold. Thanks,Nick
Expert:  Marty replied 1 year ago.

Hi Nick, the first thing I would do is a fuel mixture adjustment. If the mixture is adjusted too lean it could cause the hard starting problem. The cold start valve only helps the starting and it only sprays for a second or two. If the car takes longer than that to start and if it runs poorly until it warms up I doubt that the valve is the problem. Fuel pressure is probably ok since you have good pump pressure. I'd start with a mixture adjustment and see if it helps. Cold starting is controlled by the EHA valve located on the side of the fuel distributor. You need to make sure there is power to the valve or you won't have any cold enrichment. Power comes from the CIS control unit and the control unit is powered by the overvoltage protection relay. I think you will find that your problem is with the mixture and the EHA control.

To disconnect the alarm just remove the glove box and unplug the two plugs on the alarm control unit. It has nothing to do with the starting problem.

Let me know if you have any questions.

Marty